The recent increase in small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, about the size f garment buttons, and generally have diameters ranging up to about 1.0 inch, and heights ranging up to about 0.60 inches. The small size and the limited amount of active material contained in these small metal-air cells results in considerable attention being directed to improving the efficiency and completeness of the power generating electrochemical reactions occurring therein.
Metal-air cells convert atmospheric oxygen to hydroxyl in the air cathode, the hydroxyl then migrating to the anode, where it causes the metal contained therein to oxidize. Usually the anode material in such cells is comprised of zinc.
More particularly, the desired reaction in a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl, the hydroxyl being free to migrate through the electrolyte towards the anode, where oxidation of zinc may thereupon occur. Equation 1 outlines the overall reaction occurring in the air cathode of a metal air cell: EQU 4e.sup.- +O.sub.2 +2H.sub.2 O.fwdarw.4OH.sup.-
In most metal-air cells, air enters the cell through one or more ports in the cell, the ports being immediately adjacent to the cathode assembly, or being separated from the cathode assembly by an air chamber. In either arrangement, air diffuses into the cathode assembly, and oxygen in the air reacts with water to form hydroxide ions.
The cathode assembly of metal-air cells generally consists of a mixture of activating chemicals supported by a current collecting substrate capable of being connected to electrical circuitry. more particularly, the activating chemicals are typically comprised of carbon and manganese dioxide, and current collecting substratum usually consists of a cross-bonded screen having nickel-metal strands woven therein, or a fine-mesh expanded metal screen.
Two models are commonly forwarded to explain the overall reaction set forth in equation 1: The adsorbed Species Model and the Free Radical Model. The chains of reactions corresponding to both of these models are set forth below:
______________________________________ Adsorbed Species Model Free Radical Model ______________________________________ O.sub.2 .fwdarw. 2O.sub.Ads O.sub.2 + e.sup.- .fwdarw. O.sub.2 .sup.-. 2O.sub.Ads + 2e.sup.- .fwdarw. 2O.sup.- .sub.Ads O.sub.2 .sup.-. + H.sub.2 O + e.sup.- .fwdarw. HO.sub.2 .sup.- + OH.sup.- 2O.sup.- .sub.Ads + H.sub.2 O .rarw..fwdarw. HO.sub.2 .sup.- + OH.sup.- HO.sub.2 .sup.- .fwdarw. Bulk HO.sub.2 .sup.- + H.sub.2 O + 2e.sup.- .rarw..fwdarw. HO.sub.2 .sup.- + H.sub.2 O + 2e.sup.- .rarw..fwdarw. 3OH.sup.- 3OH.sup.- ______________________________________
Either model's chain of hypothesized reactions results in the production of peroxides (HO.sub.2.sup.-) and hydroxyls (OH.sup.-). In metal-air cathodes, peroxide may oxidize back to water and adsorbed oxygen, and act parasitically in respect of the production of hydroxyls from oxygen in the desired reaction. This parasitic effect lowers the voltage produced by conventional metal-air cells.
In the art known heretofore, MnO.sub.2, wherein the manganese in the MnO.sub.2 compound is of valence state +4, has been used in the carbon matrix to reduce the parasitic effects attending peroxide generation in metal-air cells. The use of manganese having a higher oxidation state, such as manganese (IV) dioxide (where IV denotes valence state +4), manganese (IV) oxide, or manganese (IV) hydroxide, for the reduction of oxygen in a gas diffusion electrode is well known. The introduction of manganese (IV) compounds into the carbon matrix of an air cathode has heretofore required that very finely divided manganese dioxide particles be distributed evenly by mechanical or chemical means throughout the carbon matrix of the air cathode. Heretofore, it has generally been believed that a very finely divided form of manganese dioxide is required for optimum cathode performance.
Numerous prior art disclosures have been made suggesting methods of introducing manganese compounds having a valence state of +4 into the carbon matrix of an air cathode, including:
______________________________________ Country Patent Number Inventor/Applicant Issue Date ______________________________________ Yugo. -- Zoltowski et al 1973 U.S.A. 3,948,684 Armstrong 1976 U.S.A. 4,256,545 Deborski 1981 U.S.A. 4,894,296 Borbely et al 1990 U.S.A. 4,906,535 Hoge 1990 U.S.A. 5,032,473 Hoge 1991 ______________________________________
Zoltowski et al, in an article entitled "Carbon-air electrode with regenerative short time overload capacity: Part 1--Effect of manganese dioxide," published in the Journal of Applied Electrochemistry, volume 3, pp. 271-283 (1973), disclose the use of potassium permanganate to catalyze activated carbon, wherein most of the permanganate is reduced by the carbon to MnO.sub.2, and wherein the manganese in the MnO.sub.2 compound is of valence state +4.
Similarly, in U.S. Pat. No. 3,948,684 Armstrong discloses an admixture of potassium permanganate and activated carbon, wherein the potassium permanganate is reduced in situ by heating or by the introduction of hydrogen peroxide to form manganese dioxide.
In U.S. Pat. No. 4,256,545 Deborski discloses the use of potassium permanganate while heating cathodes to between 250.degree. and 700.degree. C. in an oxidizing atmosphere to form MnO.sub.2, Mn.sub.2 O.sub.3, and Mn.sub.3 O.sub.4.
In U.S. Pat. Nos. 4,906,535 and 5,032,473 Hoge discloses the use of potassium permanganate as a catalyst for carbon black.
In U.S. Pat. No. 4,894,296 Borbely et al disclose the introduction of finely divided gamma MnO.sub.2 particles into activated carbon. Because the gamma MnO.sub.2 used by Borbely et al is not watersoluble, extensive and expensive milling is required prior to its introduction into the metal-air cell carbon matrix.
The prior art shows that heretofore the uniform introduction of manganese or manganese oxides into the carbon matrix of metalair cell cathodes has required extensive milling prior to such introduction, or high-temperature heating or chemical treatment after such introduction.
In a metal-air cell having an air cathode containing MnO.sub.2 and constructed in accordance with the teachings of the prior art, the MnO.sub.2 particles distributed throughout the carbon matrix participate in the oxygen reduction reactions occurring therein. MnO.sub.2 particles act then not merely as chemical catalysts, but as electrochemical catalysts in metal-air cells. As a result of their electrochemical activity, the physical dimensions of the MnO.sub.2 particles change as the chemical composition of those particles changes to Mn.sub.2 O.sub.3, and possibly other species, upon cell discharge.
During non-discharge or rest periods, the particles of manganese-containing species derived from MnO.sub.2 particles during cell discharge change back to MnO.sub.2 particles as a result of oxygen oxidation reactions. Through successive discharge-rest period cycles, and as a result of the repeated cycles of particle expansion and contraction corresponding therewith, the relatively inelastic carbon matrix surrounding the MnO.sub.2 particles becomes permanently deformed, and new void spaces are created between the MnO.sub.2 particles and the carbon matrix. These new void spaces may cause the air cathode material to lose mechanical integrity because of a reduction in the internal pressure exerted on the Teflon.RTM. gasket interposed between the air cathode and the cell container. In consequence of the reduction in cell internal pressure, the Teflon.RTM. seal may rupture, and electrolyte may leak from the cell. Metal-air cells having air cathodes containing MnO.sub.2 particles may therefore have a shorter service life than would otherwise be indicated by the electrochemical capacity of the cell; the service life of such a cell may be attenuated by mechanical failure of the teflon seal before the electrochemical capacity of the cell is exhausted.
Therefore, it is an object of the present invention to introduce catalytically active manganese compounds uniformly and easily into the carbon matrix of an air cathode.
It is another object of the present invention to introduce into the carbon matrix of an air cathode a manganese-containing water-soluble compound that will reduce in situ to form catalytically active non-soluble manganese-containing compounds without any heat or additional chemical treatment steps.
It is yet another object of the present invention to introduce into the carbon matrix of an air cathode a manganese-containing water-soluble compound that will reduce in situ to form catalytically active but essentially electrochemically neutral manganese compounds.
It is yet another object of the present invention to provide for metal-air cells having air cathodes of improved mechanical integrity and increased service life.
A still further object of the present invention is to provide for metal-air cells having high current densities and high closed circuit voltages.
Other objects and advantages will become apparent from the detailed description of the invention.